The Dragon spacecraft is being certified for manned spaceflight. If it's to replace Soyuz, though, it will have to serve as an escape pod for the crew. The current delivery missions are proving its unmanned variant as a reliable spacecraft capable of delivering the payload and surviving reentry. It seems not much is missing until its manned variant is ready and capable of bringing astronauts to ISS.

And then, if the flight was to be similar to that of Soyuz, it would dock to ISS and spend about seven months there (possibly more if Russians don't retire Soyuz from own flights), exposed to vacuum, UV, cosmic radiation, violent temperature changes, solar wind, possibly coronal mass ejections, its own corrosive propellants, and the fungus that tries to find any survivable niches on the ISS. Then it will have to be boarded, sealed, undocked, deorbited using own engines and 7-months-old propellant, then survive the reentry, deploy the parachutes and splash down safely.

Currently, the longest Dragon mission, SpX-9, lasted 36 days. How is the manned variant (to be?) tested and prepared for the long haul? Will there be a first unmanned verification flight of long duration, or will it all depend on simulations, ground tests and calculations of its long-term endurance?

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    $\begingroup$ Great question! $\endgroup$ – Organic Marble Jul 4 '18 at 0:41
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    $\begingroup$ Russia will not retire Souyz when Commercial Crew vehicles begin flights. Current plans are to fly Souyzes and american crewed ships one-by one, and to have one NASA crew member in each Soyuz as well as on russian cosmonaut in american ship. That plan is because of possibility of evacuation due to one-astronaut issue (heath, etc.). ISS needs at least one NASA and one Roscosmos crew members to operate. So the evacuation of one srew ship should not leave only-NASA or only-Russian crew on ISS. I can't find source now, as far as I remember it was in spacenews.com $\endgroup$ – Heopps Jul 4 '18 at 8:56
  • $\begingroup$ The are many unmanned spacecrafts operating sucessful for years "exposed to vacuum, UV, cosmic radiation, violent temperature changes, solar wind, possibly coronal mass ejections, its own corrosive propellants". $\endgroup$ – Uwe Jul 4 '18 at 11:20
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    $\begingroup$ @Uwe: ...and at current time, two manned. One made by Russia, one by China. Also, there are much, much more unmanned spacecraft that don't operate anymore. Some of them failing after their designated EOL time, some way early. $\endgroup$ – SF. Jul 4 '18 at 11:42
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    $\begingroup$ @Uwe To give a recent example, nobody at ESA is sure what exactly went wrong with Philae during its years in space. Two failures of two separate landing systems might be acceptable when deorbiting an unmanned probe on a low-gravity comet, but not when deorbiting humans on Earth. $\endgroup$ – Philipp Jul 4 '18 at 15:29

There are basically 3 concerns for long duration space flight when attached to a space station:

  • Power
  • Orbital Debris
  • Stability of maneuvering fuel

Power was the limiting factor of the Space Shuttle, which could only survive on orbit for maybe 3-4 weeks.

Fuel Stability is an issue with Soyuz, because the thruster fuel, hydrogen peroxide, tends to break down after several months on orbit, and is no longer reliable enough after about 7 months on orbit.

For the commercial crew, both spacecraft have power, and stable fuel. The issue has actually been the third, orbital debris strikes. In order to keep the probability of mission success high enough, they have to keep the spacecraft alive for months when subject to an environment of orbital debris. They followed standard precautions for orbital debris protection, increasing the protection to the point where they meet the requirements for long duration space flight.

  • $\begingroup$ There's also durability of parts in space environment. Will all the valves and seals work as well as new after 7 months in space? Lubricants evaporating, rubbers getting brittle, oxidation from oxidizer, UV weakening plastics... $\endgroup$ – SF. Mar 27 at 22:39
  • $\begingroup$ Fair enough, but those things can be tested fairly easily on the ground, and are pretty well known. The things I listed are what has been the limiting factor on all human rated spacecraft to visit the station, including the two in development in the US. $\endgroup$ – PearsonArtPhoto Mar 28 at 12:35

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